Remove duplicate words from a string (C#)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Excercises
{
public class PhoneBook
{
Dictionary Unique = new Dictionary();
StringBuilder builder = new StringBuilder();
public void removeDups(string input)
{
//Split the string into words separated by a space (' ')
string[] myStr = input.Split(' ');
//traverse the array
for (int i =0;i (less than) myStr.Length; i++)
{
//if the word already exists as a key in the dictionary, just add the count other wise add the count as 1 and the key
if (Unique.ContainsKey(myStr[i]))
{
Unique[myStr[i]]++;
}
else
{
Unique.Add(myStr[i], 1);
}
}
//for every key in the dictionary, add that to the stringbuilder if the word is uniqie. If the word is not unique add only 1 key
foreach (var d in Unique)
{
if (d.Value == 1)
{
builder.Append(d.Key);
builder.Append(' ');
}
else
{
builder.Append(' ');
builder.Append(d.Key);
builder.Append(' ');
}
}
Console.WriteLine(builder.ToString());
}
}
class Program
{
static void Main(string[] args)
{
PhoneBook Book = new PhoneBook();
string str = "This is a test sentence for my test purposes only. I am a test resource";
Book.removeDups(str);
Console.ReadKey();
}
}
}